A pacemaker is an implantable medical device that recognizes various arrhythmias such as an abnormally slow heart rate (bradycardia) or an abnormally fast heart rate (tachycardia) and delivers electrical pacing pulses to the heart in an effort to remedy the arrhythmias. An ICD is an implantable device that additionally recognizes atrial fibrillation (AF) or ventricular fibrillation (VF) and delivers electrical shocks to terminate fibrillation. Pacemakers and ICDs detect arrhythmias by sensing internal electrical cardiac signals using leads implanted within the heart. The internal signals comprise an intracardiac electrogram (IEGM). Within the IEGM, the normal contraction of atrial heart muscle tissue appears as a P-wave whereas the normal contraction of ventricular muscle tissue appears as an R-wave (sometimes referred to as the “QRS complex”). More specifically, the P-wave corresponds to the electrical depolarization of atrial tissue and the R-wave corresponds to the depolarization of ventricular tissue. The subsequent electrical repolarization of the ventricular tissue appears within the IEGM as a T-wave. Strictly speaking, P-waves, R-waves and T-waves are features of a surface electrocardiogram (EKG or ECG). For convenience, the terms P-wave, R-wave and T-wave are also used herein to refer to the corresponding internal signal component.
AF is a type of atrial tachycardia wherein the atria of the heart beat chaotically. During an episode of AF, patients often feel heart palpitations, fainting, dizziness, weakness, shortness of breath and angina pectoris (chest pain caused by a reduced blood supply to the heart muscle). Though not life threatening, AF can be quite unpleasant for the patient and so it is desirable to prevent AF from occurring. Moreover, the irregular beating of the atria during AF interferes with the proper hemodynamic function of the heart by preventing the ventricles from filling properly. As a result, optimal ventricular pressure is not achieved during each heartbeat and overall cardiac performance is degraded, i.e. the ventricles do not efficiently pump blood into the circulatory system. The ventricular rate may become somewhat erratic as well, due to conduction from the atria to the ventricles, possibly triggering a ventricular tachyarrhythmia. Furthermore, during AF, blood tends to pool in the heart chambers, increasing the risk of a blood clot forming inside the heart. Once formed, a blood clot can travel from the heart into the bloodstream and through the body, potentially becoming lodged in an artery, possibly causing a pulmonary embolism or stroke. Hence, steps are preferably taken to prevent the occurrence AF and, should an episode of AF nevertheless occur, it is deemed advisable, at least conventionally, to terminate the episode as soon as possible.
One technique for attempting to prevent AF from occurring is “overdrive pacing” wherein an implantable cardiac stimulation device, such as a pacemaker or ICD, applies electrical pacing pulses to the atria at a rate somewhat faster than the intrinsic atrial rate of the patient. It is believed that overdrive pacing is effective for at least some patients for preventing AF for the following reasons. A normal, healthy heart beats only in response to electrical pulses generated from a portion of the heart referred to as the sinus node. The sinus node pulses are conducted to the various atria and ventricles of the heart via certain, normal conduction pathways. In some patients, however, portions of the atria also generate electrical pulses referred to as “ectopic” pulses. Each pulse, whether a sinus node pulse or an ectopic pulse, has a refractory period subsequent thereto, during which time heart tissue is not responsive to any electrical pulses. A combination of sinus pulses and ectopic pulses can result in a dispersion of the refractory periods within the atria which, in turn, can trigger AF. By overdrive pacing the atria at a uniform rate, the likelihood of the occurrence of ectopic pulses is reduced and the refractory periods within the heart tissue are rendered uniform and periodic. Thus, the dispersion of refractory periods is reduced and the risk of AF is thereby also reduced. With overdrive pacing in the atria, it is desirable to achieve a high percentage of overdrive paced beats so as to reduce the likelihood of ectopic beats.
A particularly effective overdrive pacing technique for the atria, referred to herein as dynamic atrial overdrive (DAO) pacing, is described in U.S. Pat. No. 6,519,493 to Florio et al., entitled “Methods and Apparatus for Overdrive Pacing Heart Tissue Using an Implantable Cardiac Stimulation Device”, which is incorporated by reference herein. With DAO, the overdrive pacing rate is controlled to remain generally uniform and, in the absence of a tachycardia, is adjusted upwardly or downwardly only occasionally in response to breakthrough sinus beats. The aggressiveness of overdrive pacing may be modulated by adjusting the overdrive pacing rate and related control parameters. See: U.S. Pat. Nos. 6,968,232 and 7,006,868, both of Florio et al., entitled “Method And Apparatus For Using A Rest Mode Indicator To Automatically Adjust Control Parameters Of An Implantable Cardiac Stimulation Device”, and both filed Mar. 6, 2002; U.S. patent application Ser. No. 10/043,781, also of Florio et al., entitled “Method And Apparatus For Dynamically Adjusting A Non-Linear Overdrive Pacing Response Function”, filed Jan. 9, 2002; and U.S. Pat. No. 6,904,317, of Falkenberg et al., entitled “Method And Apparatus For Dynamically Adjusting Overdrive Pacing Parameters”, filed Jan. 9, 2002. Capture of overdrive pulses may be verified as set forth in U.S. Pat. No. 7,062,327, of Bradley et al., entitled “Method And Apparatus For Providing Atrial AutoCapture In A Dynamic Atrial Overdrive Pacing System For Use In An Implantable Cardiac Stimulation Device”, filed May 2, 2002.
Thus, atrial overdrive pacing, particularly DAO, provides a useful technique for helping to prevent the onset of AF. Should an episode of AF nevertheless occur, cardioversion is typically employed to terminate the episode, i.e. strong electrical shocks are delivered to the atria in an attempt to revert the atria from fibrillation to a normal sinus rhythm. Typically, each cardioversion shock delivers about two joules of energy to the atria. Cardioversion techniques are described in U.S. Pat. No. 6,445,949 to Kroll, entitled “Implantable Cardioversion Device with a Self-Adjusting Threshold for Therapy Selection”.
Although cardioversion is generally effective in terminating AF, in many cases fibrillation soon resumes, requiring another round of shocks. Repeated shocks are quite painful to the patient and can deplete battery resources of the implanted device. One reason cardioversion shocks are painful is that the patient is typically conscious and alert at the time the shock is administered. This is in contrast with stronger defibrillation shocks provided for terminating ventricular fibrillation (VF), which are typically not administered until the patient has lost consciousness. Because AF is not usually immediately life threatening, painful shocks for its treatment may be perceived by patients as worse than the disease itself and therefore not tolerated. Indeed, anxiety arising in a patient from the fear of receiving multiple, painful cardioversion shocks may be sufficient to raise the heart rate sufficiently to trigger such shocks.
As some patients have hundreds of AF episodes annually, it is desirable to provide techniques for pacing the heart during AF in such a way that the adverse effects of AF are mitigated without requiring delivery of cardioversion shocks. In particular, it is desirable to pace the heart during AF so as to improve hemodynamic performance to thereby reduce adverse symptoms suffered by the patient and reduce the risk of blood clots. It is to these ends that aspects of the invention are generally directed.
One technique that has been proposed for treating AF without cardioversion is to apply overdrive pacing to the ventricles during an episode of AF. That is, atrial overdrive pacing is applied in an attempt to prevent an episode of AF from occurring, but should one nevertheless occur, atrial overdrive pacing is deactivated and ventricular overdrive pacing is instead performed. Unlike atrial overdrive pacing, where it is generally desirable to achieve a high percentage of overdrive paced beats, ventricular overdrive pacing is preferably performed to achieve rate smoothing, i.e. to reduce or eliminate any significant changes in ventricular rate occurring over short periods of time. One advantage of stabilizing the ventricular rate during AF is that it reduces the likelihood of a more serious ventricular tachyarrhythmia be reducing conduction from the atria to the ventricles. A particularly effective technique for performing ventricular overdrive pacing for the purposes of rate smoothing is “dynamic ventricular overdrive” (DVO), which is described in U.S. patent application Ser. No. 10/456,060, to Park et al., entitled “System And Method For Dynamic Ventricular Overdrive Pacing”, filed Jun. 6, 2003, and which is incorporated by reference herein. Note that, with DVO, the ventricular overdrive rate need not be faster than the intrinsic ventricular rate, i.e. “overdrive” pacing in the ventricles can be used to actually reduce the ventricular rate (i.e. to “underdrive” the ventricles). For generality herein, the term “dynamic overdrive/underdrive pacing” is used to refer to a pacing procedure wherein pacing is delivered at a rate selected to permit detection of at least some intrinsic pulses and wherein the rate is automatically and selectively controlled in response to the detected intrinsic pulses to achieve desired overdrive and/or underdrive pacing. (Note that, underdrive pacing is sometimes more narrowly defined as “a method for terminating certain tachycardias by means of slow asynchronous pacing at a rate that is not an even fraction of the tachycardia rate”. This narrow definition is not used herein. Rather, underdrive pacing is broadly defined herein as pacing the heart (or selected chambers thereof) so as to achieve a rate generally lower than the intrinsic rate that would otherwise be exhibited.)
Although the use of dynamic ventricular overdrive/underdrive pacing, particularly the techniques set forth in the aforementioned application to Park et al., has been found to be effective for mitigating some of the problems caused by AF, considerable room for improve remains. In particular, although the rate smoothing achieved by dynamic ventricular overdrive/underdrive pacing may be effective in stabilizing the ventricular rate so as to reduce the risk of a ventricular tachyarrhythmia, smoothing of the ventricular rate does not necessarily serve to achieve optimal ventricular hemodynamic performance during each individual heart beat. Hence, overall cardiac performance can still be significantly degraded during AF. Moreover, blood can still pool in the ventricles, causing risk of blood clots.
Accordingly, it would be desirable to provide improved techniques for pacing the ventricles—particularly for use during AF—that serve to improve the pumping effectiveness of the ventricles during each individual heart beat so as to improve overall hemodynamic performance. It is to this end that aspects of the invention are particularly directed.